This invention generally relates to forming a thin-walled can, such as a can for an electrochemical cell, or an anode can for a zinc air button cell.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material.
When a battery is used as an electrical energy source in a device, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
One example of a battery is a zinc air button cell. The container of a zinc air button cell includes an anode can and a cathode can; the anode can and the cathode can are crimped together to form the container for the cell. During use, oxygen, which is supplied to the cathode from the atmospheric air external to the cell, is reduced at the cathode, and zinc is oxidized at the anode.
It is often desirable to prepare cells using thin-walled anode cans, so additional active material can be added to the cell, so as to increase cell performance
The current thickness of most anode can side walls used in current production is approximately 4 mils, i.e. 0.004 inch. It is desired to use even thinner anode can side walls so that the amount of active materials can be maximized within the interior of the zinc air button cell. A process and materials therefore need to be developed that allows thin sheets of anode can material to be stamped, shaped and formed to create anode can button cell side walls having a thickness from 0.0035 inch to less than one thousands of an inch.
A new method has been developed that allows the formation of cans having cylindrical wall and end (top) wall material thickness of 0.0035 inch, 0.0025 inch or 0.00010 inch or less thickness. A key discovery is the use of xe2x80x9cslow formingxe2x80x9d of the cylindrical wall by initially including a relatively large radius between the cylindrical wall and the reversed recessed interior walls, as the recessed interior walls are being formed. Sizing stages typically follow, which allows the distance between the reversed recessed interior walls and the outside cylindrical wall of the anode can to be gently decreased, preventing cracks in the extremely thin (and usually brittle) material. These cans may be used for battery components, such as anode cans for zinc air button cells.
Another aspect of the invention is the temporary formation of a tapered interior wall during the interior wall formation process, which is formed by use of a tapered punch.
In the forming process, the initial large radius is xe2x80x9cslow formedxe2x80x9d, and the tapered interior wall is thereafter formed as the punch decreases the distance between the recessed interior wall and the outside cylindrical wall of the anode can.
This xe2x80x9cslow formingxe2x80x9d process results in lower residual stresses in the can side walls. This process will also allow the length of the recessed interior walls to be maximized. If these cans are used as anode cans in a zinc air battery, larger zinc air button cell sizes (13 and 675) can now be formed with thinner materials having thicknesses of 0.0035 inch and below, which was previously thought impossible.
This new can forming process also has the advantage of minimizing any potential damage (scratches, etc.) to the inner (copper) layer of the interior of the can, so as to reduce and/or eliminate any potential for gassing in a battery cell, if the can is used as an anode can for a button cell hearing aid battery.
In a preferred method of forming the particular (anode battery) can, the method can be seen to comprise the steps of forming a hollow can cylinder such that the hollow can cylinder consists of a cylindrical wall, and an end wall which closes one end of the cylindrical wall. A portion of this can cylinder is reverse drawn such that the end wall is displaced toward (and preferably through) the interior of the cylindrical wall to form a recess having recessed interior walls.
During this reverse drawing process, the xe2x80x9cslow formingxe2x80x9d method of the invention is practiced wherein an initial large radius is formed in a radiused side wall transition portion (FIG. 5), followed by the formation of the tapered interior wall section as mentioned previously.
After the formation of the recessed interior walls, the distance between portions of the recessed interior wall and the outer cylindrical wall are decreased. In a preferred embodiment, the recessed interior wall is urged toward the cylindrical wall, though it is well recognized that in alternative embodiments the cylindrical wall may be urged toward the interior wall or both walls may be moved toward one another. The distance between the walls may be decreased sufficiently so that the walls touch, either temporarily, or permanently.
A portion of the cylindrical wall may thereafter be trimmed away from the existing remaining portion of the cylindrical wall to leave a relatively smooth edge, if needed. During this trimming step, the can cylinder may be turned over prior to trimming the portion of the cylindrical wall.
The invention, of course, also includes any battery can(s) formed by use of this xe2x80x9cslow formingxe2x80x9d process. More specifically, the invention would cover a battery can having a side wall thickness of less than 0.0035 inch but greater than 0.0005 inches, after the material has been formed into the battery can.
More specifically, the invention would cover material having a side wall thickness of less than 0.003 inches but greater than 0.0005 inches. More specifically, the invention would cover material having a side wall thickness of less than 0.0025 but greater than 0.0005 inches. More specifically, the invention would cover material having a side wall thickness of less than 0.0025 inches to 0.001 inches.
The actual material used for the (anode battery) can can be that, for example, as described in co-pending U.S. patent application Ser. No. 09/878,748, filed Jun. 11, 2001 included in its entirety herein.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.